In the past, various systems have been devised for automatically changing the light-transmitting characteristics of a welding helmet lens assembly in response to the existence of a welding arc. In such systems, the welder can readily see through the lens assembly under ambient light conditions when a welding arc does not exist; however, when a welding arc is struck, the lens assembly automatically becomes substantially opaque so that the eyes of the welder are protected from the intense light of the arc. An automatic lens assembly of this type eliminates the necessity for continually flipping the helmet from a raised position down to a protective position where the lens covers the eyes of the welder before an arc is struck between the electrode and the workpiece. One such automatic lens assembly is shown, for example, in Gordon U.S. Pat. No. Re. 29,684 where the lens assembly comprises a liquid crystal light shutter together with an electrical circuit which applies a suitable electrical potential to a liquid crystal film to change the same from a uniform light-transmitting condition to a uniform approximately opaque condition of very small light transmission when a welding arc exists.
In most automatic lens assemblies utilized in the prior art, the lens assembly is caused to change from a light-transmitting to an opaque condition in response to an electrical signal generated either by photocell means or a current surge through a power cable leading to the welding electrode. Sensing a surge of current in the power cable usually requires the use of a device closely adjacent the power cable which senses the surge in current, together with an electrical lead connecting the sensing device to circuitry in the welding helmet for controlling the liquid crystal light shutter. The electrical lead, however, is clumsy and can impede free movement of the welder.
A photocell which senses the welding arc, on the other hand, does not require any physical connection to the welding apparatus; however, welding helmets utilizing photo detectors are normally limited to use in buildings which are protected from sunlight. Otherwise, if the welding helmet is used outdoors, the light from the sun may trigger the photocell to cause the lens assembly to become essentially opaque when a welding arc does not, in fact, exist.